<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 200px ;
            height: 100px;
            border: solid 10px green;
            /* margin: auto; */
            /* margin-left: auto;
            margin-right: auto; */
            margin: 0 auto; 
            /* margin-top: 10px;
            margin-bottom: 10px;
            margin-left: 10px;
            margin-right: 10px; */
            text-align: center;
        }
    </style>
</head>
<body>
      <div>这是一个div</div>
      <!-- <div>这是一个div</div>
      <div>这是一个div</div> -->
</body>
</html>